This invention occurs in the field of art involving, generally, lubricating oils adapted for use between a plurality of relatively moving surfaces with which the fluid compositions are in contact for the purpose of reducing the friction between these surfaces while providing protection, concomitantly, from wear and corrosion. These fluid compositions, or lubricating oils, tend to deteriorate under conditions of use in present day diesel engines with attendant formation of sludge, lacquer and resinous materials which adhere to the engine parts; particularly the piston ring, groove and skirt, thus reducing the operating efficiency of the engine. To counteract the formation of these deposits, certain chemical additives have been found which when introduced into lubricating oils have the ability to keep the deposit-forming materials suspended in oil so that the engine is kept clean and in efficient operating condition for extended periods of time. These agents are known in the art to which this invention pertains as detergents, inhibitors, or detergent-inhibitors. Metal organic compounds are useful in this respect. One class of metal organic compounds which has been found particularly useful are the sulfurized normal and overbased calcium alkylphenolates. These agents are believed to be effective because they provide alkalinity capable of neutralizing strong organic and inorganic acids and are capable of inhibiting the formation of deposits and deposit precursors in the oil phase. Overbased and normal sulfurized metal alkylphenolates have been found to be particularly effective detergent-inhibitors in lubricating oils.
By the term "overbased" in this context is meant, generally, that the ratio of the number of equivalents of calcium to the number of equivalents of alkylphenolate moiety is greater than 1. In the present instance, the term is also used with particular reference to calcium alkylphenolates having a ratio of calcium metal to alkylphenolate moiety of at least 2.9:2. Ther term "overbased" also has reference herein to alkali metal hydrycarbyl sulfonates having, desirably, a TBN of at least 50. Many overbased sulfurized metal alkylphenolates having a calcium metal to alkylphenolate ratio greater than 1 and less than 1.7 have proven to be useful lubricant additives heretofore. Normal calcium alkylphenolates also provide useful lubricant additives. The term "normal" indicates that the ratio of the number of equivalents of calcium to alkylphenol moiety is 1. U.S. Pat. Nos. 3,528,917; 3,549,534; 3,761,414; and 3,969,325 describe lubricating compositions containing sulfurized normal calcium alkylphenolate detergent-inhibitors of improved resistance to oxidation. Illustrative of lubricant oil compositions containing sulfurized overbased calcium alkylphenolate detergent-inhibitors are those described in U.S. Pat. Nos. 3,474,035 and 3,706,632.
It has been found recently, however, that railway diesel engine oils having a high degree of alkalinity, that is, a TBN of at least 10, are particularly desirable in that they prevent corrosion by oil-soluble acids formed by oxidative deterioration at the high temperature existing under normal conditions of engine employment in proximity to the combustion chamber. The term "TBM" or "nominal TBM", as employed herein, refers to "total base number" which is defined as the quantity of acid, expressed in terms of the equivalent number of milligrams of potassium hydroxide that is required to neutralize all basic constituents present in one gram of a given sample. The method of evaluating alkalinity is that defined in ASTM Method D 664. While the foregoing alkalinity can be attained by introduction into the lube oil of a nominal 300 TBN forty percent of fifty percent overbased calcium sulfonate in a naphthenic oil carrier, the resulting lubricant compositions are unsatisfactory because these overbased materials degrade the silver protection characteristics of the oil, a factor of particular significance with respect to railway diesel engines, the vast majority of which, in the United States, and to a significant extent outside of the United States, as well, utilize silver-plated piston pin insert bearings.
Particularly useful lubricant compositions are those containing both substantially normal and highly overbased sulfurized calcium alkylphenolate and highly overbased alkaline earth metal sulfonate additives. One facet of this utility is the provision of finished lubricant oils for use in railway diesel engines having a TBN of at least 10 and thus capable of preventing corrosion by oil-soluble acids formed by oxidative deterioration at the high temperatures existing under normal conditions of engine use in proximity to the combustion chamber. Where a sulfurized naphthenic oil-containing composition (having a sulfur content by weight of at least 1 percent) is incorporated with the foregoing overbased additives, the destruction of the silver protective properties of the lubricant oil by the overbased calcium alkylphenolate is overcome, but not the similarly destructive properties of the alkaline earth metal sulfonate. Nevertheless, the incorporation of an alkaline earth metal sulfonate in these lubricant oils is most desirably because of the improved engine performance it provides over an extended period of time.
Thus, the production of a finished lubricant oil for use, particularly, in railway diesel engines having the necessary degree of alkalinity and, for this purpose, incorporating a normal or highly overbased sulfurized calcium alkylphenoate and an alklaine earth metal sulfonate, without diminution of the silver protective properties of the finished lubricant oil would provide a significant advance in the state of the art.